Buy a hard tail bike for XC or All mountain riding, avoid any full suspensions at your price range they will not do as well. The dirt jumpers are going to beat to hell, I'd avoid them as well. Buy the bike, ride some trails, got hooked, buy more bikes, ride harder trails, repeat... Start with cross country, move on from there, it can be your gateway drug. Also bargining on CL is expected, if you find something you like offer like 60% and settle about 75% of their asking, any lower and they may get insulted.